Establishing a liquid-tight connection with the interior of a vessel, such as a tank or a pipe, through a transverse hole in the vessel wall can be difficult when access to the vessel interior is limited. Unilateral plumbing fittings such as those disclosed in International Patent Application Publication No. WO 2014/085867 can be installed without access to a vessel interior. But these devices have not garnered significant commercial appeal, likely due to the connections being unreliable and/or difficult to install.
Many liquid-tight structures are flexible. For example, roofing membranes, plastic bags, etc., are formed from flexible material that holds liquid in use. It may be necessary or helpful to connect these flexible structures to drain or supply fittings in a liquid-tight manner to control the flow of liquids through the flexible material.